Twin-belt continuous-casting machines used to cast molten metal employ upper and lower endless casting belts which are relatively thin and wide. The casting belts are formed of suitable, heat-conductive, flexible, metallic material as known in the art. The upper and lower casting belts are each revolved under high tension around a respective belt carriage in a substantially oval path. The revolving upper and lower belts define a moving-mold casting region. The casting region is formed between the nominally flat casting belts traveling from the entrance of the casting machine into the casting region to the exit therefrom. Thus, the casting region extends from the entrance to the exit end of a continuous molten-metal casting machine along an ostensibly flat casting plane.
While revolving in its substantially oval path, each casting belt is in direct and intimate contact with and is continuously passed around an entrance-pulley drum and an exit-pulley drum, that are relative to the entrance and exit of the casting region. Alternatively, each casting belt may be passed around the combination of an entrance non-rotating, belt-levitating semi-cylindrical belt-support apparatus and an exit-pulley drum. The non-rotating, belt-levitating semi-cylindrical belt-support apparatus typically employs pressurized air or other fluid to float or “levitate” a casting belt allowing it to move along the stationary apparatus and revolve in its substantially oval path. The pressurized fluid is emitted from a semi-cylindrical, fluid-pillow shell that levitates the casting belt and facilitates its rotation. This apparatus and method is described in U.S. Pat. Nos. 6,386,267 and 6,575,226 respectively, hereby incorporated by reference in their entirety.
The combination of a non-rotating, belt-levitating cylindrical belt-support apparatus and an exit-pulley drum provides several advantages. The use of such a combination provides additional space within the caster which may be utilized for improved cooling, support and stabilization of the casting belts. With either combination, however, the casting belts must be tensed, guided or steered, and in some cases, preheated before entering the casting portion of the mold. These functions are discussed in greater detail below.
Casting belts are typically tensioned by moving the exit-pulley drum of the caster. Each casting belt is under significant and uniform tension across the full width of the moving mold casting region. Tensioning is generally accomplished by moving the exit-pulley drum in a direction horizontal or parallel to the casting plane.
In addition to being tensioned, both the upper and lower belts also must be steered or guided. As the caster belts revolve during caster operation, they tend to move laterally in an unpredictable manner. Caster belt steering is the inducing of an intentional transverse movement in a desired direction in order to achieve or maintain optimal tracking of the casting belt during molten metal casting. The belts cannot be steered or guided, however, by confining their lateral movement through edge guidance efforts. The lateral motion of the highly-tensioned belts around a pulley involves such large sideways or edgewise forces that an edge of a revolving belt would distort, crumple and tear against a movement-restricting edge guide.
Hence, traditionally, with the belt in direct contact with each pulley perimeter surface, the belt is steered or guided by slightly tilting the axis of rotation of the exit-pulley drum. The axis of rotation of an exit pulley drum is tilted or skewed either horizontally or vertically (or combination thereof) relative to the plane of the casting region of the belt being steered. Steering the belt by employing vertical tilting is the most effective. Horizontal and vertical tilt steering are described in greater detail below and in U.S. Pat. No. 4,901,785 which is hereby incorporated by reference in its entirety.
The horizontal-tilting, or horizontal-skew, of the axis of rotation of an exit pulley drum serves to create a very-small leading-angle in relation to the axis of rotation of the exit-pulley drum. This small leading-angle causes the belt to approach the exit pulley drum in the desired lateral-direction for controlled horizontal skew belt steering. The progress of the belt in the lateral direction on the exit-pulley drum also creates a small leading-angle of the belt return loop in relation to the axis of rotation of the entrance pulley(s) resulting in a similar controlled horizontal skew belt steering at the entrance pulley(s).
The vertical-tilting, or vertical-skew, of the axis of rotation of an exit pulley drum serves to create a very small leading-angle of the belt in relation to the axis of rotation of the exit pulley drum. Simultaneously, an associated small leading-angle of the belt is created in relation to the axis of rotation of the entrance pulley drum. In other words for vertical-skew steering of a traditional caster, the belt wraps on both the entrance pulley and exit pulleys at an angle to the plane of the pulley rotation equal to the angle of vertical offset of the exit pulley in relation to the entrance pulley.
However, substituting a non-rotating, levitating, fluid-pillow belt-support apparatus for the entrance-pulley directly interferes with both belt steering concepts. The adverse impact to entrance-end fluid-pillow caster-belt steering control derives from the absence of direct, or intimate, contact of the highly-tensed caster belt to the perimeter surface of a rotating belt support structure. As such, without direct-contact of the caster-belt to a rotating entrance-pulley surface, horizontal-skew side-to-side force-differential steering and vertical-skew lead-angle steering cannot precisely control the belt tracking.
Thus, the creative integration of narrower shoulder-pulleys into the fluid-pillow design allows for the significant advantages for both fluid-pillows and caster-steering pulleys to be realized without compromising standard belt steering capabilities.
In addition, casting belts are often preheated to ensure casting of uniformly high-quality product. Preheating a casting belt before entering the mold reduces thermally induced strains in the belt, thereby assisting in keeping the belt flat during casting. Flat belts protect the solidifying molten metal being cast from unpredictable belt distortions caused by the high temperature casting. Belt preheating is disclosed in U.S. Pat. No. 4,537,243, which is hereby incorporated by reference in its entirety.
In casters employing non-rotating, semi-cylindrical, fluid-pillow belt-support apparatus, it is feasible to both support and preheat the belt through the use of an elevated temperature pressurized fluid, e.g., air, water or steam. To safely accomplish these functions, it is important to have effective edge sealing and controlled venting of the hot pressurized fluid. Typically, the hot pressurized fluid is vented to the ambient environment. Ideally, however, the hot fluid is entrapped and contained so that it may be recovered and potentially recycled rather than vented to the surrounding environment.
In light of the above, a need exists for an effective belt steering or guiding system for a caster equipped with a non-rotating, belt-levitating semi-cylindrical belt-support apparatus at the front-end of the mold. Likewise, a need exists for a system to effectively entrap and contain hot pressurized fluid so that it may be recovered and potentially recycled. The present invention of employing rotating shoulder pulleys in combination with non-rotating belt-levitating fluid-mold entrance belt-support structures facilitates our continuing need to employ belt preheat and fulfills these requirements.